BorrowSanitizer

[tomassedovic]

Metadata
Point of contact	@icmccorm
Team(s)	compiler, infra, lang, opsem
Goal document	2026/borrowsanitizer

Summary

We are building BorrowSanitizer: an LLVM-based instrumentation tool for finding violations of Rust's aliasing model. In 2026, we want to make it feature-complete and useful in practice.

Tasks and status

---

Note: we have updated the body to match the 2026 goal. Your original text is preserved below.

Details

Metadata
Point of contact	@icmccorm
Team(s)	compiler, infra, lang, opsem
Goal document	2026/borrowsanitizer

Summary

We are building BorrowSanitizer: an LLVM-based instrumentation tool for finding violations of Rust's aliasing model. In 2026, we want to make it feature-complete and useful in practice.

Tasks and status

[tomassedovic]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[icmccorm]

May has been busy for BorrowSanitizer! We just posted our monthly status update. Here's the TL;DR on what our team has been up to:

• Our retag function calls are starting to make their way upstream, with #156208 and #156210.

• We have expanded our test suite, replicating many of the cross-language Tree Borrows errors that we found in our prior work with MiriLLI.

• We have a new benchmarking dashboard for tracking the performance of the LLVM components of BorrowSanitizer, in a "no-op" mode. This inserts our instrumentation, but it does not execute any run-time checks. This was originally a summer project by @Gitter499.

• We are using Miri to test out optimizations that we hope to bring to BorrowSanitizer. This effort has been lead by @mojeanmac in collaboration with @shinhae-kim from Cornell. We created a new allocation strategy for Miri that leads to a 6.7% speedup. We are also looking into using metadata from Tree Borrows to determine when to run Miri's garbage collector. This has lead to considerable improvements in performance—up to a 171.3% speedup (63% time reduction) for one crate.

• We posted an RFC for BorrowSanitizer's LLVM components.

[icmccorm]

Here's our June status update! TL;DR:

• Added support for Tree Borrows' wildcard provenance, expanding our test suite to cover 93% of relevant tests from Miri's pass and fail directories, up from 85% last month.
• Implemented our initial deferred reference counting garbage collector.
• Upgraded our diagnostics to point to relevant user code.

We also updated our benchmarking dashboard to compare our full instrumentation against Miri, and our retag intrinsics are nearly finished!

[icmccorm]

Here's our July status update! TL;DR:

• We optimized our garbage collector, discovering new heuristics that we can use to improve Miri, too!
• We found two new cross-language aliasing bugs! BorrowSanitizer is ready to try out on real libraries.

We plan on submitting an MCP for upstreaming BorrowSanitizer as an experimental component in nightly between now and RustConf in September.